It’s not yet clear how a quasi-legal pot industry might operate in Colorado and Washington or what its public-health effects will be. It could be that these states are harbingers of a slow, national reassessment of marijuana policy. Or their experiment could serve as warning for the other 48 states. For now, the federal government does not need to stage an aggressive intervention, one way or the other. It can wait, watch and enforce the most worrisome violations as they occur.